yeah basically that is the case. every image has it own tag. the thing is Turbine (Frosty<3) releases those tags via the skin asset pack when something new come up, but for various reasons they have never released a complete list of the tags (I presume it's because they don't want the skins to affect game play or something like that).
You can try to guess some tags or data-crawl them if you have enough motivation (like the author of Low Key Store has)
but even then if you find something out it might not work - or it might work in beta but it's locked in live version later on.
Best way is to cross your fingers and hope that Frosty will drop a new skin asset pack to us - soon.